The present invention relates to graded index optical devices and to a method of making the same. More particularly, this invention relates to a method of fabricating optical devices such as waveguides by heat treating a fluorine containing glass body to produce a change in the refractive index of a surface region of the body due to fluorine out-diffusion. As used herein, the term "optical waveguide" refers to a material containing a system of refractive index gradients capable of guiding waves of optical energy. This term, therefore, includes both optical fibers which are usually employed as the transmission medium for optical communication systems and to planar devices which are usually employed in optical circuits that are required for processing optical signals. The light propagating channel can be a cylindrical core in the case of an optical waveguide fiber, or it can be a planar layer on the surface of a substrate or sandwiched between two adjacent layers of lower refractive index. The term "optical energy" and "optical," as used herein, include the infrared, visible and ultraviolet portions of the electromagnetic spectrum.
Some operational theories and other pertinent information concerning optical waveguide fibers can be found in U.S. Pat. No. 3,157,726 issued to Hicks et al. and in the publication "Cylindrical Dielectric Waveguide Mode" by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pages 481-498, May 1961. Information concerning planar optical waveguides may be found in the publications: "Evanescent Field Coupling into a Thin-Film Waveguide" by J. E. Midwinter, IEEE Journal of Quantum Electronics, Vol. QE-6, No. 10, Oct. 1970, pages 583-590; "Light Waves in Thin Films and Integrated Optics" by P. K. Tien, Applied Optics, Vol. 10, No. 11, Nov. 1971, pages 2395-2413; and "Dielectic Rectangular Waveguide and Directional Coupler for Integrated Optics" by E. A. J. Marcatili, The Bell System Technical Journal, Vol. 48, No. 7, Sept. 1969, pages 2071-2102.
Although single mode waveguides are advantageous in that they are capable of propagating optical signals with very low dispersion, due to the low numerical aperture and/or small core size of such fibers, lasers must be employed to inject optical signals into these waveguides. Multimode waveguides generally have larger core diameters and larger numerical apertures than single mode waveguides and are therefore often the preferred medium for the transmission of optical signals, since they can accept light from incoherent, broad spectral width sources such as light emitting diodes. However, in a multimode waveguide, the various modes propagate at slightly different group velocities. Thus, a short input pulse that is shared by a plurality of guided modes splits up into a sequence of pulses that arrive at the output end of the waveguide at different times. This type of pulse dispersion is the dominant cause of dispersion in multimode waveguides.
A well-known mode equalization technique which results in decreased dispersion requires an index gradient across the light propagating core or channel. For example, assuming a cylindrical waveguide, the refractive index is greatest along the axis thereof and decreases as a certain power.alpha.of the radius. A discussion of graded index waveguides appears in the publication "Optical Fibers for Communication" by D. Gloge, Applied Optics, Vol. 13, No. 2, Feb. 1974, pp. 249-254 and "Multimode Theory of Graded-Core Fibers" by D. Gloge and E. A. J. Marcatili, Bell System Technical Journal, Vol. 52, No. 9, Nov. 1973, pp. 1563-1578. As a result of this variation in refractive index across the optical waveguide core or channel, light rays deviating from the axial direction propagate into regions of lower index where their higher speed compensates for the greater distance of propagation. Thus, as graphically illustrated on page 252 of said Gloge publication, the impulse response of graded index optical waveguides is substantially improved.
Gradient index optical waveguides are also advantageous in that they have less light scattering loss because boundary imperfections are smoothed out. In planar waveguides surface imperfections at the boundary between the light transmitting layer and air, for example, still contribute to scattering, but the modal optical power is concentrated away from the surface of the planar structure. Therefore, loss due to scattering at that surface is reduced.